Duplexers



March 14, 1961 J. REED ETAL DUPLEXERS 2 Sheets-Sheet 1 Filed Feb. 21, 1957 A JOHN P550 4 GERSHOA/J WHEELER A ORNEY March 14, 1961 J. REED ETAL 2,975,381

DUPLEXERS Filed Feb. 21, 1957 2 Sheets-Sheet 2 F76. 6 A /5 3O I l l 4 4 TRANS- MITTER 39 4 42 TD TUBg RECEIVER P L L 2- 7Q LOAD TUBE ANTENNA 40 /NVENTORS Jaw/v REED GERSHON J. WHEELER MM K QW A T mRA/EY United States Patent DUPLEXERS John Reed, Belmont, and Gershon J. Wheeler, Newton Centre, Mass., assignors to Raytheon Company, a corporation of Delaware Filed Feb. 21, 1957, S81. No. 641,552

9 Claims. Cl. 333-10 This invention relates to an improved duplexer and more specifically to a duplexer that includes as a basic part a branched guide coupler arranged and adapted in a novel manner as disclosed herein.

In the past, branched guide couplers have been constructed wherein the impedance in each branch has been arranged to be proportional to the coefficients of hinomial expansion in order to maintain high directivity. For example, a five branch coupler would have impedances in each slot or branch in the ratio of l:4:6:4: 1.

In this invention, there is disclosed a branched guide coupler of improved and novel design which may be constructed for either a coaxial line, strip transmission line or waveguide that does not follow the binomial expansion formula. In a paper entitled A Method of Analysis of Symmetrical Four-Port Networks by the inventors, published in the I.R.E. Transactions on Microwave Theory and Techniques, volume MTI4, Number 4, October 1956, there is shown a complete mathematical analysis of this invention. By utilizing the improved branched guide coupler as disclosed herein there has been produced a new type of duplexer that consists of only a pair of TR tubes and at least a three-section branched guide coupler. In the art as practiced today duplexers are constructed of at least a first four-arm hybrid coupler connected to a pair of TR tubes which in turn is connected to a second four-arm hybrid coupler. Many forms of the aforementioned duplexers are in use today, their most important advantage being the elimination of the ATR tubes.

In this invention, a TR tube is placed in each main guide section between that branch which forms the guide for the transmission of power between the transmitter and the antenna. In operation the magnetron power will fire the TR tube, thereby effectively preventing power from being transmitted through the coupling, but rather allowing said transmitted power to be fed to the antenna. Since a load member is placed in the fourth arm of the coupler, which is on a diagonal from the transmitter, any leakage power from the transmitter would be coupled into the load as in a regular balanced duplexer. Upon reception of an echo from the transmitted pulses, the TR tubes would fire, and hence the full power of the echo pulse would be transmitted through the coupler to the receiver which is connected on the diagonal opposite the antenna, thereby elfectively coupling all the received power into the receiver arm. The branched guide coupler having at least three branches is used, since it has been discovered that said coupler will couple power incident on any arm into the arm diagonally opposite it. Thus, all power incident on the first arm will go into the fourth arm, and all power incident on the second arm will go into the third arm.

The duplexer, as described herein, has certain size advantages in the longer wavelengths of the microwave spectrum. For example, an L-band duplexer with hybrids would be about twelve inches wide, nine inches high, and about three feet long, whereas a duplexer as "ice described in this invention would be at most six inches long, three or four inches high, and about one foot wide, since the low power element could be in a coaxial line. These advantages in dimension would be even more pronounced at P band, thereby giving a price advantage and a reduction in the complications of construction and specifically those of the hybrid circuits. The coaxial components are simple to make, and further, the TR windows would be carrying only the full line power and not \/2 times the line power, as is the case in the normal hybrid circuits as used today.

Further objects and advantages of this invention will be made more apparent as the description progresses, reference now being made to the accompanying drawings wherein:

Fig. 1 illustrates a branched guide coupler having four branches or slots;

Fig. 2 is a side view of the branched guide coupler illustrated in Fig. 1;

Fig. 3 is a line drawing of a two-section branched guide coupler;

Fig. 4 is a line drawing of a three-section branched guide coupler;

Fig. 5 is a three section branched guide coupler similar to that illustrated in Fig. 4 showing how improved bandwidth is obtained by controlling impedance levels of the main guides;

Fig. 6 is a three-section branched guide coupler in coaxial line; and

Fig. 7 illustrates a duplexer utilizing a three-section branched guide coupler as an essential element.

Referring now to Figs. 1 and 2, there is shown a branched guide coupler 10 consisting of a first main waveguide 11 and a second main waveguide 12 connected together along their broad walls by means of a plurality of branched guides 13, 14, 15 and 16. Main guide 11 forms part of a microwave transmission line and is structurally connected and electromagnetically coupled to the second waveguide 12 by means of branched guides l3, 14, 15 and 16, which are relatively short and make T-junctions with the wider walls of the main and secondary guides 11 and 12, the longitudinal axis of the component branched guides being co-planar. Each waveguide 11 and 12 may be rectangular in cross-section, as shown, and dimensioned to operate in the dominant (TE mode in which the electric field vectors of the energy involved extend perpendicularly to the broad walls. In terms of the relative direction of the E-fields (electric fields) within the guides, each junction formed in the illustrated connection of waveguides may be termed an E-plane T. The impedances of the guides forming such a T are effectively in series, and for this reason, the interconnecting-branched guides may be termed seriesbranching guides. As will be noted from Figs. 1 and 2, the distance between inside connecting walls of waveguides 11 and 12 is a quarter wave length, as is the distance between center points of the interconnecting branch guide couplers 13, 14, 15 and 16. The impedances of the branch guides are controlled by varying the distances between opposing walls of and in a similar manner the impedances of the main guides are controlled by varying the distance between opposing broad walls ex tending perpendicularly to the axis of the main guide. In this manner, it is possible to construct a coupler having any characteristic impedance in any arm of the guide. It has been discovered that, by controlling the independent characteristic impedances of the main guides and the branch guides, it is possible to control the coupling and directivity characteristics and bandwidth characteristics of the coupler.

Referring now to Fig. 3, there is shown a two-section branched guide coupler in coaxial line form having the ass-mam required parameters, which are that the distance between the main lines 17 and 18 is a quarter wave length apart, as is the distance between branched lines 19 and 20. By assuming the characteristic admittance of the input line at point 21 to be Y =l, and the characteristic admittances in main lines 17 and 18 between the branched lines 19 and 20 to be Y zb, and further, that the characteristic admittances of lines 19 and 20 are both equal to a, it has been determined that the device will be matched and perfectly directed if l+a =b Thus, the coupling into arm 22, which is on a diagonal from input arm 21, will be:

20 log For the special case where a=1 and b=\/2, the device is a 3 db directional coupler since power fed into input 21 will divide equally between output arms 22 and 23.

Referring now to Fig. 4, there is shown a three-section directional coupler in coaxial form having the same required one-quarter wave length spaced-apart distances, as shown in Fig. 2. It has been discovered that if three branch arms are used, it is not necessary to change the admittances of the main lines in order to achieve perfect match and directivity. For example, if the characteristic admittances of all the main lines is considered to be Y,,=1, and the characteristic admittances of the two outer branch guides 24 and 25 are Y -c, then it has been determined the perfect match will occur when and that coupling into the output arm, which is on a diagonal from the input arm, will be:

20 log It should be noted that when a=c=l, that the coupling is zero db which signifies there is no loss in power from the input to the diagonal output arm.

A broad band device in a three-arm branched guide coupler can be achieved when the characteristic admittances of the main lines are changed, as shown in Fig. 5, where the characteristic admittancesof all the main lines are Y b, and the input characteristic admittance is maintained equal to 1. For match and perfect directivity it has been discovered that the following condition must exist:

For this condition, coupling into the output arm, which is on a diagonal from the input arm, will be:

20 log This equation shows that for a 3 db coupler a= /21: Considering now a unit signal entering input arm 26,

it can be shown that power division and directivity and match will be dependent on the parameters a, b, and c,

and that condition for good match is:

As long as this condition holds true, the power will divide between output arms 27 and 28 and will be dependent only on a as follows:

2 1 2 Power at P r mas-( 2 s Owe a: a +1 In order to construct a directional coupler having any desired coupling K, it is necessary that the following equation be satisfied:

Since this equation has two values for a, for any desired coupling K, it has been found that if a value for a less than unity is used, the coupler will have high directivity and good match over a broader frequency band than if the higher value of a is used.

It has also been discovered that in a coupler with five or six cross arms, there could be a third difierent size for the center arms; however, if the center arms are all kept at the same value, then for the special case of equal power distribution (3 db coupler) in the two output arms, the admittances are indicated in the following table:

This table indicates that for the four arm waveguide directional coupler, illustrated in Figs. 1 and 2, the i111- pedances of arms 13 and 16 illustrated therein will be equal and havea value of 0.2346 times the impedance of the input arm, and that the two center arms 14 and 15 will be equal and have a value of 0.5412 times the impedance of the input arm.

Referring now to Fig. 6, there is shown a three-section branched line coupler having branch arms 29, 30 and 31. The coupler illustrated in Fig. 6 must satisfy the basic requirement that the main lines 32 and 33 be spaced a quarter wave length apart, and further, that the spacing between the branched lines also be a quarter wave length. The impedance characteristics of either the main lines 32 and 33 or 34 and 35, or any of the branched lines 29, 30 or 31 can be controlled in a conventional way, which is to change the physical dimensions of the inner conductor with respect to the outer conductor, and, in this way, it is possible to obtain the same degree of coupling and directivity for any of the configurations disclosed in Figs. 1, 2, 3 and 4.

Referring now to Fig. 7, there is shown in three-section branched guide coupler consisting of branch guides 36, 37 and 38 connecting main guides 39 and 40. Main guide 39 is constructed of a first guide 41 and a second guide 42, which contains a TR tube. In a similar manner, main guide 40 is constructed of a first guide 43 and a second guide 44 which contains a second TR tube. The spacing requirements between the main guides 39 and 40 are the same as those described previously, which is that the distance between said guides must be a quarter wave length, and also the distance between branch guides must be a quarter wave length long, as illustrated. Connected to the guide closest to the TR tube in guide 42 is a transmitter 45, and connected closest to the TR tube in guide 44 is an antenna 46. Located diagonally opposite transmitter 45 is a load device 47, and located diagonally opposite antenna 46 is a receiver 48. In actual operation, firing of a transmitted pulse from transmitter 45 will cause the TR tubes in guides 42 and 44 to fire, thereby preventing any energy from being propagated or fed back to the receiver 48 or load 47. It is entirely conceivable that, due to low efficiencies of currently available TR tubes, a certain amount of leakage energy will be fed therethrough. In any event, energy that does couple from transmitter 45 will see a three-section branched guide coupler which will couple the energy into load 47. Firing of the TR tubes will thereby cause the energy from said transmitter to be fed directly through guide 38 to the antenna 42. Upon the receiving of an echo or incoming wave energy to antenna 46, the TR tubes will appear as matched lines, and therefore the energy from said antenna will see a three-section branched guide directional coupler which will therefore couple all the energy to receiver 48.

This completes the description of the particular embodiments of the invention illustrated herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. Accordingly, it is desired that this invention not be limited by the particular details described herein, except as defined by the appended claims.

What is claimed is:

l. A directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and three spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacing of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, and the impedance of said outer branch paths being equal to each other and less than the centrally located branch path.

2. A directional coupler comprising a first wave guide and a second wave guide arranged and adapted to carry electromagnetic energy, said first and second wave guides being substantially rectangular in cross-section and dimensioned to operate in a dominant mode, said first and second wave guides spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and at least four spaced-apart branch wave guides connecting said first and second wave guides, each of said branch wave guides forming an E-plane junction with said first and second wave guides, the lengths and the center-to-center spacing of said branch wave guides being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, the centrally located branch wave guides having a greater distance between opposing broad walls perpendicular to the axis of said first and second wave guides, the lengths and the center-to'center branch wave guides to provide a broadband coupling of electromagnetic energy between said first and second wave guides.

3. A directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and three spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacing of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, and the admittance of said center branch path being greater than that of either of said outer branch paths to provide a refiectionless coupling between said first and second energy paths.

4. A directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and three spaced-apart branch paths connecting said first and second energy paths, the lengths and center-to-center spacing of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, the magnitude of the coupling from the input to the output arm in the ratio of and for perfect match the following relationship should exist:

Fri

where a is the characteristic admittance for shunt junctions and impedance for series junctions of the both outer branch paths and c is the characteristic admittance for shunt junctions and impedance for series junctions of the center branch paths, normalized to the admittance of the input line, the admittance of said branch paths being proportional to the values a and c.

5. A directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and three spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacing of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, the magnitude of the coupling from the input to the output arm in the ratio of 20 log where a is the characteristic admittance for shunt junctions and impedance for series junctions of the both outer branch paths, and b is the characteristic admittance for shunt junctions and impedance for series junctions of the main energy paths existing between each of said branch paths, and c is the characteristic admittance for shunt junctions and impedance for series junctions of the center branch path, normalized to the impedance of the input line, the impedance of said branch paths being proportional to the values a and c.

6. A directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, means for generating wave energy connected at one end of said first energy path and means for receiving wave energy connected at the other end of said first energy path, means for radiating said generated wave energy connected at one end of said second energy path and a dissipative load absorbing device connected at the other end of said second energy path, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, three spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacing of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, the impedance of said outermost branch paths being equal to each other and less than the impedance of the centrally located branch path, a first TR tube in said first main path adjacent to said branch path nearest to said generating means and a second TR tube in said second main path adjacent to said branch path closest to said radiating means.

7. A directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, means for generating wave energy connected at one end of said first energy path and means for receiving wave energy connected at the other end of said first energy path, means for radiating said generated wave energy connected at one end of said second energy path and a dissipative load absorbing device connected at the other end of said second energy path, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, three spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacing of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, a first TR tube in said first main path adjacent to said branch path nearest to said generating means and a second TR tube in said second main path adjacent to the branch path nearest to said radiating means, and where a=b==c=l where a is the characteristic admittance for shunt junctions and impedance for series junctions of the both outer branch paths, and b is the characteristic admittance for shunt junctions and impedance for series junctions of the main energy paths existing between each of said branch paths, and c is the characteristic admittance for shunt junctions and impedance for series junctions of the center branch path, normalized to the admittance of the output line.

8. A three db directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and three spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacings of said branch paths being substantially an odd multiple of a quarter wave length at the operating frequency of said coupler, the characteristic admittance for shunt junctions of both the outer branch paths being equal and of magnitude of substantially 0.4141 and the characteristic admittance for shunt junctions of the center branch paths having the magnitude of substantially 0.07071.

9. A three db directional coupler comprising a first energy path and a second energy path arranged and adapted to carry electromagnetic energy, said first and second paths spaced apart a distance substantially equal to an odd multiple of a quarter wave length at the operating frequency of said coupler, and four spaced-apart branch paths connecting said first and second energy paths, the lengths and the center-to-center spacing of said branch paths being substantially an odd multiple of a quarter Wave length at the operating frequency of said coupler, the characteristic admittance for shunt junctions of both the outer branch paths being equal and the characteristic admittance for shunt junctions of the center branch paths being of greater magnitude than that of the outer branch paths.

References Cited in the file of this patent UNITED STATES PATENTS 2,512,673 Page June 27, 1950 2,558,385 Purcell June 26, 1951 2,586,993 Riblet Feb. 26, 1952 2,640,915 Sichak June 2, 1953 2,701,340 Miller Feb. 1, 1955 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent N05 2,975,:ae1 March 14 1961 John Reed et altv It is hereby certified that error appears in the abov ent requiring correction and that the sa corrected below; v

e numbered patid Letters Patent should read as Column 3, line 29, after "determined'! for "the" read that column 5, line 45, for the lengths and the centerto-center'" read thereof than either of the outside Signed and sealed this 1st day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,975,381 March 14, 1961 John Reed et a1.

It is hereby certified that error appears in the above numbered patent reqiiring correction and that the sa id Letters Patent should read as corrected below.

Column 3, line 29, am that column 5, t0center" read the after "determined" for "the" read line 45, for the lengths and the centerreof than either of the outside Signed and sealed this 1st day of August 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

